Precise evaluation of corrosion environments of structural materials under complex water flow condition, (II) - Water chemistry at downcomer region of a BWR

In order to clarify the corrosion environment of nuclear reactor structure material precisely, the water radiolysis evaluation considering a flow of reactor water was performed. This study focused on the downcomer region of a Boiling Water Reactor (BWR). Radiation intensity of the region is high and water radiolysis reaction occurs well. Therefore, the region is recognized as important for recombination of oxygen, hydrogen peroxide and hydrogen back to water under hydrogen water chemistry (HWC) condition. Precise evaluation of water chemistry in the region with consideration of water flow, however, has not been performed. In the downcomer region water flow is very complicated because the jet pumps are close together and water flows out to the recirculation system through only two nozzles. A new water radiolysis model was developed which can simulate water radiolysis under the three-dimensional water flow condition. The calculation was performed using the result of water flow analysis obtained by the computer code "alpha-Flow." The water chemistry near a shroud wall and the reactor pressure vessel (RPV) wall differed greatly, and especially so under HWC condition. Also, the detailed distribution of the electrochemical corrosion potential (ECP) of structural materials was clarified by using the water chemistry analysis result.